JPS5819594A - Control rod element for reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear reactor control rod element, and particularly to a nuclear reactor control rod element suitable for a light water-cooled or heavy water-cooled nuclear reactor.

The following will explain the control rod of a boiling water reactor as an example.

In a boiling water reactor, one control rod is installed in every four fuel assemblies, excluding the area around the core, to control the power distribution. FIG. 1 shows a horizontal sectional view of four fuel assemblies and one control rod. In FIG. 1, 1 is a fuel assembly consisting of a large number of fuel rod bundles and a channel box surrounding the fuel rod bundles, and 2 is a cross-shaped control rod blade for bundling control rod elements 3 together.

FIG. 2 shows a longitudinal sectional view of the control rod element. The control rod element is constructed by enclosing boron carbide (B, C) powder in a vibrating manner in an absorber tube 31 made of stainless steel.

A plenum 33 is provided below the absorber tube 31 to store helium gas generated when B and C stay in the reactor. The density of B4C powder is about 70% of the theoretical density.

When a control rod configured in this way is used in a nuclear reactor, B,
10B force contained in about 15% in C (4
With H6? It splits into Li, and the 10 B concentration in B and C gradually decreases, and the control rod value also decreases accordingly. Figure 3 shows the relative % IIJ rod value decline with respect to the burn-up of toH. Normally, when control rod power decreases by about 10%, the control rod is considered to have reached the end of its lifespan from a nuclear standpoint.

In addition to the above, the following factors can be considered as factors that determine the service life of control rods. When the above-mentioned control rod is used in a nuclear reactor for a long period of time, the B and C powders become concentrated and gradually form a pellet-like sintered body having a shape that matches the inner surface shape of the absorber tube. The process of forming pellet-like sintered bodies proceeds most rapidly near the upper tip of the control rod element, which is irradiated with a large neutron flux. As IOH burns, B and C undergo swelling, and the swelling rate becomes particularly large after the pellet-like sintered body is formed. Figure 3 shows the relationship between the volumetric expansion coefficients of B and C and the burnup of IOB. The swelling of B and C is not favorable for the apsono tube. Part of the swelling of B and C is absorbed by the voids in the absorber tube. However, if the control rod remains in the reactor for a longer period of time, After the void inside the absorber pipe is filled, B, which is trapped inside the absorber pipe,
Due to the swelling of C, pressure is generated that tries to expand the absorber pipe from the inside, and there is a possibility that distortion will occur in the absorber pipe.

Therefore, the control rods are usually removed from the reactor before excessive strain occurs in the absorber tube due to the swelling of B and C. For this reason, the control rod usage period was approximately half of its nuclear lifespan.

Absono due to swelling of B and C (as a means to prevent excessive distortion of the pipe, patent application No. 110380/1989)
No. 1, it is stated that a hollow tube is provided within the B and C powders in a part of the control rod element in the axial direction, and the swelling of the B4C powder is absorbed by the crushing of the hollow tube. However, providing a hollow tube does little to prevent swelling of the B and C powders, and also prevents large pressure from acting on the absorber tube immediately after the B4C sintered body is formed. It is not possible. Furthermore, since the crushing of the hollow tubes inside the B and C powders requires an extremely large pressure of 4C, it is thought that the possibility of complete crushing occurring is small. Therefore, by providing only a hollow tube, the effect of extending the mechanical life of the absorber tube is small, and a significant extension of the life can not be expected.

An object of the present invention is to provide a control rod element that can be used in a nuclear reactor until the control rod reaches the end of its nuclear life without causing excessive strain in the absorber tube even if swelling occurs in B4C. It's about doing.

In order to achieve the above object, the present invention (1) 84C powder is sealed in a cylindrical thin metal container having an outer diameter smaller than the inner diameter of the absorber tube, and the absorber tube is filled with the metal container. A gap is provided between the thin-walled metal container and the absorber tube, and the gap width is configured to increase closer to the upper tip of the absorber tube, or (
2) Fill the absorber tube with cylindrical or cylindrical shaped bodies of B and C powders with different outer diameters, create a gap between the B4C sintered pellets and the absorber tube, and adjust the gap width to the upper tip of the absorber tube. The objective is achieved by increasing the size as it approaches , or (3) by combining methods (1) and (2) above. That is, according to the configuration (1) above, the B and C powders are sintered and shaped to match the inner shape of the thin metal container during use in the nuclear reactor,
As a result, a gap is formed between the sintered bodies B and C and the absorber tube to absorb the swelling of B and C. the above(
With the configuration 2), a space capable of absorbing the swelling of B and C is provided from the beginning between the shaped bodies B and C and the absorber tube. With the control rod element configured as described above, even if swelling occurs in B4C during use in a nuclear reactor,
, C sintered body or B, due to the gap existing between the C shaped body and the absorber tube or formed during irradiation, B,
It is possible to absorb the swelling of C and prevent excessive strain from occurring in the absorber tube, allowing the control rod to be used in a nuclear reactor until the control rod reaches its nuclear life.

In addition, in the method (1) above, the B and C powders are sintered and shaped to match the inner shape of the thin metal container, and in the method (2) above, a cylindrical or cylindrical shaped B and C body is formed in advance. Therefore, there is no fear that the B and C sintered bodies will be deformed due to neutron irradiation, and the value of the control rod will not deteriorate near the tip of the control rod element.

FIG. 4 shows an example of the axial burnup distribution of control rod elements. The burnup at the upper tip of the control rod element is larger than other parts, and the local burnup at the tip governs the service life.

Hereinafter, the present invention will be explained in detail with reference to the embodiments shown in FIGS. 5, 6, and 7. In the embodiment shown in FIG. 5, a plurality of thin metal containers 41 filled with B and C powders 42 are sealed in the upper part of the absorber tube 31, and B and C powders 32 are filled in the lower part of the container as in the conventional case. The structure is such that it is directly sealed into the absorber tube. The thin metal container 41Fi has a cylindrical shape and is made of stainless steel, for example. Further, micropores are provided between the inner space and the outer space of the thin metal container 41 to provide ventilation. FIG. 8 shows the relationship between the outer diameter of the thin metal container 41 and the free volume provided within the absorber tube. As can be seen in Figure 4, the burnup of B and C is greater as it approaches the upper tip of the control rod element, and therefore the swelling rate of B and C is greater, so the outer diameter of the thin metal container used is The closer the control rod element is to the top tip, the smaller it becomes.

In that case, the outer diameter of the metal container is as shown in Figure 8.
Care should be taken to provide a gap that can absorb the swelling of B and C according to the target burnup of 60B. for example,
As shown in FIG. 3, in order to absorb the swelling of B4C when the control rod is used until its nuclear life in the gap provided between the metal container 41 and the absorber tube 31,
At least 30.5 cm measured from the top of the absorber tube
Use a thin-walled metal container with an outer diameter of about 85% of the inner diameter of the absorber tube in the length range, an inner diameter of about 80-83% of the inner diameter of the absorber tube, and a diameter of at least 30.5 cr11 measured from the top of the absorber tube. or more and at least 91
．． Approximately 89 mm of the absorber pipe inner diameter for distances of 4 m or less
% outer diameter and an inner diameter of about 84-87%, at least 91.4 m measured from the top of the absorber tube and at least 182.9 CrrI.
The following distances use a thin metal container having an outer diameter of about 94% and an inner diameter of about 89 to 92% of the inner diameter of the absorber tube. As shown in Figure 4, the amount of neutron irradiation at the lower part of the control rod element is lower than that near the upper tip, so even if B and C powders are directly sealed in the absorber tube as in the past, No problem. As for the axial length of the metal container 41, it is preferable that the metal container 41 enclosed in the upper part of the absorber tube has a short length, and the metal container 41 located in the lower part has a longer length. This is because in the process of forming a sintered body of B and C by irradiation, the volume of B and C may temporarily shrink due to densification, and if the volume shrinks temporarily, neutrons Near the top tip of the control rod element, where the bundle is high and is important in determining the value of the control rod, there is no loss of B and C, and B and C can be distributed uniformly in the axial direction. It's for a reason.

In the embodiment shown in FIG. 6, shaped bodies 43 of B and C powders having cylindrical or cylindrical shapes having different outer diameters are enclosed in the absorber tube 31. The outer diameters of the shaped bodies B and C are made smaller as they are located in the upper part of the absorber tube, and larger as they are located in the lower part.

However, the outer diameter of the B4C shaped body is '. It is determined to provide a gap that can absorb the swelling of B4C corresponding to the target burnup of B. FIG. 9 shows the relationship between the outer diameter of the B4C shaped body and the free volume within the absorber tube.

For example, at least 30 mm measured from the top of the absorber tube.
．． In the length range of 5cr11, the outer diameter of the B and C shaped bodies is approximately 84% of the inner diameter of the absorber pipe, and the B4C shaped body is used at a distance of at least 30.5crr1 or more and at least 91.4crn or less from the upper end of the absorber pipe. The outer diameter of the body is approximately 88% of the inner diameter of the absorber pipe, and the outer diameter of the shaped body B and C is equal to the inner diameter of the absorber pipe at a distance of at least 91.4 ar1 and at least 182.91 m1 from the upper end of the absorber pipe. Approximately 93.5%. With this configuration, even if the control rod element is used in a nuclear reactor until the local burnup of the IOB near the upper end of the control rod element reaches about 85%, that is, until the nuclear lifetime is reached. , the swelling of 9 B and C can be sufficiently absorbed by the gap provided at the initial stage. Note that the shaped body of B4C powder is manufactured by mixing, for example, graphite with B and C powders and sintering the mixture at high temperature. The shaped bodies B and C manufactured in this way do not lose their shape at the temperature during use in a nuclear reactor and can maintain their original shape.

The embodiment shown in FIG. 7 is an improvement on the embodiment shown in FIG. 4 or 5. In the embodiment shown in FIGS. 4 and 5, the lower part of the apson tube is directly connected to B and C.
It was configured to enclose powder. In the embodiment shown in FIG. 6, a thin metal hollow tube 51 is installed in the part where the B and C powders are directly sealed in the absorber tube, and a metal receiving plate is placed above the tube to support the B and C sintered pellets. 52 is installed.

The thin-walled hollow tube 51 and the receiving plate 52 are made of stainless steel, for example, and the B, C shaped body 43 or B
, has enough strength to sufficiently support the metal container 41 containing the C powder, and is made as thin as possible. With this configuration, when the control rod element is used in a nuclear reactor, even if the volume of the B4C powder sealed in the lower part is temporarily reduced, the B4C shaped body or B4C powder sealed in the upper part It is possible to prevent the powder-filled metal container from moving downward. Even if B and C sealed in the lower part are sintered into the shape determined by the absorber tube 31 and the thin-walled hollow tube 51, and the swelling of B4C increases thereafter, the collapse of the thin-walled hollow tube 51 causes the B , C swelling can be absorbed and is expected to have the effect of preventing excessive strain from occurring in the absorber pipe.

In the embodiments shown in FIGS. 5, 6, and 7, it is necessary to remove the heat generated in B and C during use in a nuclear reactor, but the absorber tube and metal container 41 Alternatively, the larger the gap width between the B and C shaped bodies 43, the lower the heat transfer coefficient and the higher the temperatures of B and C. However, if the control rod elements are filled with 1 atm of helium in advance, the increase in B and C temperatures due to the provision of the gap width is at most about 20 to 30C.

The outer diameter of B and C at the upper part of the control rod element is approximately 7
~15% smaller size may reduce control rod value. However, if the density of the shaped bodies B and C is increased to about 85% or more of the theoretical density, the number density of 10B will increase compared to the conventional one, which will reduce the value of control rods that occur during long-term use. The ratio can be made smaller than in the past, resulting in less variation in control rod value over the life of the rod.

In the embodiment shown in FIG. 5, in order to reduce the rate of deterioration in the value of the control rod that occurs during long-term use, the composition ratio of boron B and C sealed in a metal container 41 with a small outer diameter is changed, Approximately 15% more than conventionally used 10B
Increase concentration. By doing so, the rate of deterioration in control rod value that occurs during long-term use can be reduced to the same level or lower than that of conventional control rods.

As explained above, according to the present invention, even if control rods are used in a nuclear reactor for a long period of time until they reach their nuclear lifespan, excessive strain does not occur in the absorber tubes, and the control rods have a long service life. can be extended to the maximum extent possible, resulting in significant improvements in terms of safety and economy.

[Brief explanation of the drawing]

Figure 1 is a horizontal sectional view of the fuel assembly and control rod blades of a boiling water reactor, and Figure 2 is a longitudinal sectional view of control rod elements.
Figure 3 shows control rod value, B 4 C volumetric expansion coefficient, and 10
FIG. 4 shows the axial position of the control rod element and the burnup range; FIGS. 5, 6, and 7 are longitudinal sectional views of the control rod element of the present invention; FIG. 8 is a diagram showing the relationship between the outer diameter of the thin-walled metal container and the free volume inside the absorber pipe, and FIG. 9 is a diagram showing the relationship between the outside diameter of the shaped bodies B and C and the free volume inside the absorber pipe. DESCRIPTION OF SYMBOLS 1... Fuel assembly, 2... Control rod pre, -d, 3...
... Control rod element, 31 ... Absorber pipe, 32, 42
... B, C powder, 33 ... plenum, 41 ... thin-walled metal container, 43 ... B4C shaped body, 51 ... thin-walled cylindrical tube, 52 ... metal tar [ノ til -1= J =====) 2121st % 3 Figure IOδn A et al. tfr Oτtas L cliff (×)th
4 Figure'l115if

Claims (1)

[Scope of Claims] (1) A control rod element for a nuclear reactor comprising B and C as neutron absorbing substances sealed in a metal cladding tube, in which B4C powder has different outer diameters and heights, and an inner air volume and an outer space. The control rod is sealed in a cylindrical thin-walled metal container with a ventilation hole in between, and B
, C were sealed in the thin-walled metal containers so that their outer diameters and heights were arranged in order from small to large, and B and C powders were directly sealed in other parts of the control rod cladding tube in the axial direction. A control rod element for a nuclear reactor featuring: 2. In the nuclear reactor control rod element described in claim 1, instead of the thin metal container in which B and C are sealed,
Formed bodies of B and C powders that have been previously shaped into cylindrical or cylindrical shapes with different outer diameters and heights and do not lose their shape are shaped from one end of the control rod cladding tube to the other. A nuclear reactor control rod element characterized in that the elements are enclosed so as to be arranged in order from largest to largest. 3. In the control rod element for a nuclear reactor as set forth in claim 1 or 2, a circle is formed at one end in the axial position where B4C powder is directly sealed in the control rod cladding tube without passing through a thin metal container. A thin-walled metal container or B, C in which a thin-walled hollow cylindrical metal tube with a plate-shaped metal plate attached is arranged and B4C powder is enclosed.
A nuclear reactor control rod element characterized by having a structure capable of supporting a shaped powder body. 4. In the control rod element for a nuclear reactor according to claim 1, 2 or 3, the boron composition of B and C is determined by the inner diameter of a thin metal container containing B4C powder or B, A control rod element for a nuclear reactor, characterized in that the 10B concentration of boron in the shaped body of C powder is higher than the 10B concentration of boron in 84C powder. 5. In the control rod element for a nuclear reactor according to claim 1, 3 or 4, the outer diameter of the thin metal container is at least 30 mm as measured from the upper tip of the control rod element.
．． At a distance of 5 cm or less, approximately 8 of the inner diameter of the absorber
5%, at least 30.5m or more, and 91.4
At a distance below cry+, the inner diameter of the absorber pipe is approximately 8
9%, with at least 91.4 and 182.9
Approximately 94% of the absorber pipe inner diameter at a distance of less than cm
A control rod element for a nuclear reactor, characterized by: 6. In the control rod element for a nuclear reactor as set forth in claim 2, 3, or 4, the outer diameter of the shaped body of B, C powder is at least equal to At a position at a distance of 30.5 crr1 or less, it is approximately 84% of the absorber pipe inner diameter, and at a position at least 30.5 crr1 or more and a distance of 91.4 m or less, it is approximately 88% of the absorber pipe inner diameter, and at least 91.4 cIn or more. 1
A control rod element for a nuclear reactor, characterized in that the inner diameter of the absorber tube is about 93.5% at a position at a distance of 82.9 crn or less. 7. Claims 1, 3, 4 or 5
2. The control rod element for a nuclear reactor according to item 1, wherein the height of the thin-walled metal container is made smaller at the upper tip of the control rod element.